(1) Field of the Invention
The present invention relates to an apparatus for intermittently driving an endless belt in an automatic screen printing machine. More particularly, the present invention relates to an apparatus for intermittently driving an endless belt in an automatic screen printing machine, in which the precision of feeding of the endless belt is enhanced irrespectively of non-uniform mechanical properties of the endless belt which are due to the uneven thickness and other factors.
(2) Description of the Prior Art
As the apparatus for intermittently driving a printing endless belt by a direct current electric motor, there has been known an apparatus comprising a direct current electric motor for intermittently driving an endless belt, a switch for setting a repeat length of the endless belt while converting it to a pulse number, a pulse generator for detecting as a pulse number the actual feed length of the endless belt according to displacement of the endless belt, a digital display device for displaying the repeat length detected by said detecting mechanism in the form of a numerical figure, a digital control mechanism for generating a starting signal, an acceleration signal, a constant speed signal or a speed reduction and stop signal according to the pulse number set by said switch and an electric motor control mechanism for controlling an input to said direct current electric motor according to the signal from the digital control mechanism to start the electric motor, accelerate the electric motor, drive the electric motor at a constant speed or decelerate and stop the electric motor, wherein said digital control mechanism comprises a computing mechanism for subtracting the detected pulse number from the set pulse number and generating a speed reduction signal so that the endless belt is stopped at a repeat length corresponding to the set pulse number [see Japanese Patent Application Laid-Open Specification No. 3448/79 (Japanese Patent Publication No. 24427/80)].
In this intermittent drive apparatus, the mechanical feed precision is excellent. However, the feed error due to the endless belt per se is indispensable because the endless belt is ordinarily composed of a reinforced rubber or the like and is not a rigid member.
This feed error of the endless belt is due to non-uniform mechanical properties owing to the uneven thickness and other factors. Especially in case of intermittent feeding, the feed error is caused by the difference of the quantity of elongation at the respective starting times and the difference of the degree of contraction at the respective stopping times. Furthermore, in the case where of two supporting rollers, only the pulling roller is driven to start the endless belt as a whole at a stroke from the stopped state as in conventional techniques, the tension is concentrated on the pulling side of the endless belt, which is another cause of occurrence of the feed error of the endless belt.